1. Field of the Invention
The present invention relates generally to a process for the manufacture of porous solids and, more specifically, to a process for the manufacture of solid, high thermally conductive porous graphite artifacts and to improvements in the manufacturing process to enhance the properties of the artifacts so formed.
2. Description of the Prior Art
This invention deals with carbon in its various forms and, particularly to artifacts formed of solid, porous graphite. Carbon fibers have been used commercially in industry for many years. Carbon fibers are known to exhibit extraordinary mechanical properties due to the unique graphitic morphology of the extruded filaments. Advanced structural composites have been created which advantageously exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix.
Additionally, many of the current applications of carbon fibers have evolved from structural reinforcement applications to thermal or heat sink applications. For example, heat sinks have been utilized in the aerospace industry to absorb energy in applications such as missiles and aircraft where rapid heat generation is found. A number of heat absorption applications are also envisioned for the automotive industry.
These and other applications have stimulated research into novel reinforcements and composite processing methods for carbon materials. Acceptable materials must exhibit high thermal conductivity, low weight and a low coefficient of thermal expansion, among other requisite properties.
POCO Graphite, Inc., of Decatur, Tex., assignee of the present invention, has previously produced a line of specialty graphite materials that are routinely used in a wide range of highly technical and industrial applications. The following grades of porous graphites have been produced:
These high strength, fine grained isotropic graphites are easily machined by conventional machining methods. Their high strengths and small particle sizes allow the fabrication of complex components containing tight tolerances. The isotropic nature of the materials provides uniform electrical and thermal properties.
In spite of these advantages, the bulk thermal conductivities of these porous solid graphites have generally been below about 100 W/mK with apparent densities of 1.9 g/cc and below. Efforts have been undertaken to produce porous graphite materials which exhibit even higher thermal conductivities in order to meet present and future commercial expectations.
Attempts have been made to improve upon the properties of solid graphite materials through the production of pitch based carbon xe2x80x9cfoamxe2x80x9d materials. The apparent densities of such materials are lower than the apparent densities of the specialty graphites listed above. For the most part, the previously described prior art foam processes also resulted in foams which exhibited low thermal conductivities, generally less than about 58 W/mK.
One attempt to produce an improved carbon xe2x80x9cfoamxe2x80x9d is described in now issued U.S. Pat. No. 6,033,506, issued Mar. 7, 2000 to Klett and in issued U.S. Pat. No. 6,037,032, issued Mar. 14, 2000, to Klett et al. The processes described in the Klett patents included steps which were less time consuming than the earlier known techniques for producing graphite foams and offered the potential to lower production and fabrication costs. Perhaps more importantly, the Klett process claimed to produce carbon foams with thermal conductivities, generally greater than 58 W/mK.
Although the Klett process was an improvement in pitch based carbon foaming processes, the Klett process utilized a static pressure during the formation of the green artifact (billet). Routinely, this static pressure selected was about 1000 psig. Graphite foams made in this manner have shown significant density gradients, generally ranging from about 0.25 g/cc at the top of a production billet to about 0.60 g/cc at the bottom of the billet and have exhibited voids and cracks. The claimed thermal conductivities have also not been achieved in some instances.
Applicant""s own improvements to the original Klett process, described in co-pending application Ser. No. 09/862,560, filed May 22, 2001, entitled xe2x80x9cProcess For Making Carbon Foam Induced By Process Depressurizationxe2x80x9d, use a xe2x80x9cflashxe2x80x9d method to induce boiling of the pitch precursor and produce porous graphite xe2x80x9cfoamsxe2x80x9d which have apparent densities ranging between 0.40-0.65 g/cc and thermal conductivities exceeding 58 W/mK without exhibiting voids or cracks as in the initial Klett process.
A need continues to exist for graphite artifacts having even higher thermal conductivities, for example greater than 70 W/mK. A need thus exists for a solid, high thermally conductive porous graphite with an apparent density which exceeds that of the previously described graphite foams and with thermal conductivity characteristics greater than 70 W/mK.
A need exists for an improved method for producing artifacts having these characteristics which artifacts are substantially free of density gradients, voids and cracks.
It is one object of the present invention to provide a solid, high thermally conductive porous graphite which has a more uniform density gradient profile with less tendency to crack as a finished product as compared to the prior art.
Another object of the invention is to provide such a solid, porous graphite with an apparent density greater than about 0.678 g/cc.
Another object of the invention is to provide such a solid, porous graphite which has a thermal conductivity greater than 70 W/mK.
In a specifically preferred process of the invention for producing a porous graphite, pitch is introduced into a mold, the pitch having a characteristic boiling point at a given pressure and for a given temperature. Air is then purged from the mold. The pitch is then pressurized between a preselected initial processing pressure and a relatively lower final processing pressure. The preselected initial pressure serves to increase the boiling point of the pitch above the boiling point at the final processing pressure. The pitch is heated while at the initial processing pressure to a temperature below the solidification point but above the boiling point which typically occurs at the final processing pressure. The pitch is then depressurized from the initial processing pressure to the final processing pressure while maintaining the process temperature above the typical boiling temperature at the final pressure to thereby produce a porous artifact. The porous artifact is heated to a temperature that solidifies and cokes the porous artifact to form a solid, porous carbon. The solid, porous carbon artifact can then be cooled to room temperature with simultaneous release of pressure. The porous carbon artifact then undergoes additional heat treatments to produce a porous graphite artifact having a thermal conductivity greater than 70 W/mK and a density greater than graphite foam.
The preferred solid, porous graphite artifacts so produced have a thermal conductivity greater than 150 W/mK and a density greater than 0.678 g/cc. Artifacts having thermal conductivities greater than 150 W/mK have been produced having densities in the range from 0.678 g/cc and 1.5 g/cc.
Additional objects, features and advantages will be apparent in the written description which follows.